Sequence stratigraphy of the middle to upper jurassic, viking graben, north sea

Abstract

The Middle to Upper Jurassic in the Viking Graben area was deposited during an overall transgression. Seven second order regressive/transgressive (R-T) facies cycles can be identified in the Jurassic from the Lower Toarcian to the base of the Cretaceous. Three cycles occur during a minor rift phase and four during a major rift phase. The regressive phase of the cycles occurs during periods of most active fault rotation. These R-T facies cycles are the dominant control on facies distribution. The R-T facies cycles correlate throughout the Viking Graben area regardless of the local tectonic setting or the local sediment source and supply, and appear to be synchronous. Second order cycles strongly influence the development of superimposed third order sequences. During a second order regression, shelfal areas and local highs are often eroded. The geometry of the shelf break and sediment supply influence the way the cycles develop. Where sediment supply is high and the slope is steeply dipping, thick, coarse-grained turbidites develop during third order lowstands. When slopes are steep, no lowstand prograding wedges develop. On gently dipping slopes, lowstand prograding wedges develop without thick turbidites. Where sediment supply is low, third order sequences are poorly developed, but the second order regression is marked by an upward increase in silt content and an upwards decrease in anoxic shales. During second order transgressions, previously eroded areas are flooded and deposition is more widespread on the basin margin. Slope geometry and sediment supply are important to the development of the third order cycles. Thick, stacked, shoreface sandstones may develop on terraces or on gently dipping slopes if sediment supply is high. The bases of these sequences often show an abrupt basinward shift in facies followed by backstepping facies. Turbidites develop during third order lowstands when there is a steeply dipping slope and ample sediment supply. When sediment supply is low, third order sequences are poorly developed, but the second order transgressions consist of alternating shale and siltstone with the shale content increasing upward. Transgressions may be capped by anoxic shales. Local tectonics and local sediment supply are important components in the development of basin stratigraphy, but do not appear to be the cause of these R-T facies cycles. Local tectonics control the geometry and angle of the slope. Steep fault escarpments prevent the development of shallow water progradation past the slope break, and are a source of coarse material in turbidites and debris flows. Fault activity probably contributes significantly to the accumulation of debris near the fault escarpment. R-T facies cycles appear to be related to regional tectonics and may also be related to changes in eustacy caused by major tectonic events (tectono-eustacy). These R-T facies cycles provide a powerful framework for regional correlation and for the prediction of regional facies distribution over a wide area when tectonic setting and sedimentation rates are variable.